Photographic printer circuit



1951 A. E. GLANDON ETAL 2,995,978

PHOTOGRAPHIC PRINTER CIRCUIT Filed Sept. 26, 1958 5 Sheets-Sheet 1 2022) y g CONTROL U PHOTOTUBE e & L 24 ER vnunee 12: m

POWER n 11 Pl.

n C NETWORK 32 34 so 10 K K LOGARITHMIC s suMMme momma NETWORK TAMPUFIERFj. g. 1

AdrianEGIandOn Roscoe H.6'anad INVENTORS ATTORNEYS Aug. 15, 19612,995,978

A. E. GLANDON ETAL PHOTOGRAPHIC PRINTER CIRCUIT Filed Sept. 26, 1958 5Sheets-5118M. 2

AdrianllGlandon RoscoeHflanaday INVENTORS Aug. 15, 1961 A. E. GLANDONETAL 2,995,978

PHOTOGRAPHIC PRINTER CIRCUIT Filed Sept. 26, 1958 5 Sheets-Sheet 3 fiFig: 11

PHOTUTUBE e Z LOGARITHMIC PHOTOMETER Gf c EA l Q3 E195 E E 0: E8 :m aAdrianE.Glandon I E RosooeHL'anaday i3 E INVENTORS (00 E W M 1 i BYfiinfw.

I ATTORNEYS NEGATIVE DENSITY United States Patent The present inventionrelates to the automatic'control of printing exposure in the printing ofcolor negatives, and more particularly concerns the recognition andcompensation of certain errors in conjunction with such printing.

The automatic control of printing exposure in making color prints fromcolor negatives, or transparencies, is

well known. In a typical exposure control system, a print-- ing lamptransmits light through the negative, and a predetermined fractionalportion of the light passing through the negative is directed onto aprinting surface. The remaining fractional portion of the light isdirected onto a phototube, whose output signal is compared with apreselected reference signal. The difference between the two signalsconstitutes a comparison signal and represents the degree to which astandard, or normal," exposure must be changed for producing an opitimumprint from the particular negative. The exposure'may be changed byvarying the exposure time or' the lamp intensity or both. In theprinting system embodying the present in vention, the printing timeremains constant a nd the lamp intensity is adjusted to produce theoptimum exposure.

Exposure control systems of the above type maybe rendered automatic byusingthe comparison signal as an error signal in a servo system, orloop, which adjusts the lamp intensity in such a direction and by suchan amount as to reduce the error signal to zero. The regulation of thephototube output to produce a zero error signal is accomplished byregulating the light input to the phototube to a standard value. .Sincethe phototube receives a predetermined fraction of the light transmittedthrough the negative, and since the remainder of that light falls on theprinting surface, the light intensity at the printing surface isregulated to a standard value, or aimpoin by the servo loop. It is wellknown that the log of the intensity of the light transmitted through anegative is proportional to the density of the negative. Therefore it isconvenient to refer to the aimpoint of a printer in terms of the logintensity of the printing light.

When a printer of the type dmcribed above is employed for printing thethree primary colors sequentially, or for otherwise separatelyregulating the printing exposure of each of these colors to a standardvalue, the integrated densities of the three colors in the print may bemade equal to each other, in which case the print is said to be balancedto gray. Balancing the color densities in this manner has been found toproduce acceptable color prints in a large percentage of a randomlyselected group of negatives. However there are some negatives which,when printed by the above method, produce unacceptable prints. Variousfactors have been found to contribute to the occurrence of thelast-named group of negatives, and include: (1) excessive contrastbetween the densities of various portions of the negative; (2) excessivedeparture from color balance in the overall photographed scene; and (3)non-uniform response in the three primary colors, with respect tovarious amounts of exposure of the original negative.

The first two of the above three factors depend upon the nature andlighting of the photographed subject and are independent of the negativedensity for any color.

Each of these two factors may be compensated by increice mentallyadjusting the aimpoint of the printer, i.e., by adjusting the logintensity of the light at the printing surface by an amount which issome fractional increment of the log intensity of the standard aimpoint.The compensations for these two factors are referred to, respectively,as negative classification and color correction. The third factorreferred to above depends upon the integrated density of the negative.Its compensation, referred to as slope control, is achieved by varyingthe aimpoint of the printer as a function of the integrated negativedensity.

In accordance with the present invention the aimpoint of the basic servosystem is shifted for negative classification, color correction andslope control during the serial printing of each of'the three primarycolors. The aimpoint is shifted by adjusting the operating point of the'phototube which constitutes an element of the servo corrective factors.

' tion of the density of the negative that is to be printed.

A further object is to modify the aimpoint of a servo system byadjusting the operating point of a phototube which constitutes anelement of the system. 1

More specifically, it. is an object to modify the aim- A point of aservo system by adjusting the dynode voltage of a photomultiplier tubewhich constitutes an element of the system. i

Other objects of the invention will appear from the followingdescription, reference being made to the accompany-ing drawings,wherein:

FIG. 1 is a block diagram of the basic servo system of the exposurecontrol system, as modified by the corrective input circuits of thepresent invention;

FIG. 2 is a schematic diagram of the details of the control phototubeof- FIG. 1;

FIG. 3 is a block diagram of the n+c network of FIG. 1;

FIG. 4 is a schematic diagram of the circuit of FIG. 1, showing ingreater detail the n+0 and s networks; and

FIG. 5 is a graph showing the operation of the printer control with andwithout the introduction of the correction factors n+0 and s.

Basic control system No. Q,794,366. The basic control system, shownsche- 1 ma-tically in FIG. 1, includes a printing lamp 10 whichilluminates a negative, or transparency 12 and projects an imagethereof, by means of a lens system indicated generally at 13, through abeam splitter 14 and a printing filter 16 onto a sheet 18- of printingmaterial. The beam splitter 14 directs a predetermined fraction of theprinting light through a monitor filter 20 onto a control phototube 22.Suitable lenses (not shown) may be supplied in the light paths, in awell known manner, to cause proper focussing on the printing paper 18and on tube 22.

The voltage output of the control phototube 22 is compared to areference voltage E the source of which is connected to the phototubeoutput through a load resistor R The difference between the two comparedvolt-- ages, designated the error signal e, constitutes the controlinput signal for an amplifying circuit comprising a voltage amplifier 24in series with a power amplifier 26. The power amplifier, in turn,drives printing lamp 10. The circuit comprising lamp 10, negative 12,phototube 22 and amplifiers-24 and 26 constitutes the basic servo loop,or system, which stabilizes itself in a well known manner by varying thelamp intensity in such direction and amount as to reduce the value .ofthe error signale toward zero. When the error signal substantiallyequals zero, the voltage output of the control phototube 22 is constantand virtually equals E therefore, the log intensity of the light inputto phototube 22 also must be substantially constant. Since beam splitter14 directs a fixed fraction of the printing light onto phototube 22 andthe remaining fixed traction of that light onto the printing sheet 18,the light intensity on the print sheet,

or the aimpoint of the control system, is maintained virtually constantby the basic servo system.

Expressed algebraically the operation of the basic servo system is asfollows:

log H-d==log h'+log h" v 1 where log -h"=log m dh'=K 2 But the relativetransmission-and reflection of the beam which holds the splitter is alsoconstant, so that the light transmitted to the printing paperisconstant:

Control phototube The control phototube 22 employed in the system is amultiplier phototube, or photomultiplier tube of the well known typeshown in FIG. 2, having a photosensitive cathode 21, a series ofintermediate secondarily emissive electrodes 23, called dynodes, and ananode 25. The anode 25 is connected through load resistor R to source Eof anode reference potential and is connected to an output terminal 29.The successive segments 27 of a potential divider connect the successivedynodes to stepped voltage sources, illustrated in FIG. 2 as extendingbetween a maximum dynode voltage E and a reference voltage, ground.

The relatively high sensitivity of the multiplier phototube, when usedas the control phototube 22 (FIG. 1), makes possible the use of a beamsplitter 14 having a high ratio of transmission to reflection. The beamsplitter herefore transmits onto the printing paper most of the lighttransmitted by the negative 12.

'Phototube 22 is required to control the light intensity at which theservo system stabilizes for a given negative density. Therefore, somemeans must be provided to change the operation point of that phototube,preferably without affecting the gain of the basic servo system, whileas a matter of design the systemgain must be consistent withrequirementsforstability, accuracy of regulation,

and degree of rejection of extraneous influence on the circuit. Thetransfer characteristic of the multiplier phototube is given by where z:anode current, in amperes, v

=cathode sensitivity, in amperes per lumen, =current amplification fromcathode'to anode, in amperes per ampere, h=light intensity at cathode,in foot candlw, A=cathode area, in square feet.

The current gain, 7, is dependent on dynode voltage, the relationshipbeing exponential in character:

'Y= d where:

a=a constant; 1 E =dynode voltage; and m=a constant.

Combining Equations 6 and 7,

i =C'aE h"A (8) When the phototube is operated in a system whichregulates to 'keep its anode current nearly constant,

where E =referenoe voltageapplied toanode load resistor; R =value ofanode load resistor; and e=error voltage output.

The error voltage e approaches zero as the closed-loop servo systemreaches balance. At balance,'-for practical purposes,

E =i R ==R CaE Ah" 10) For a given circuit and phototube type, R C, aand A are constants, and Equation 10 may be written E =kE h" 11 wherek=R CocA Thus, a change in dynode voltage causes a change in the lightintensity at balance. 7

The transfer gain of the phototube, as employed in the self-regulatingsystem, may be defined as the rate of change of error output voltagewith respect to log intensity light input:

phototube transfer gain (12) .i log h Since the output voltage (i R is alinear function of the input light intensity, a given percentage inlight input will cause an equal percentage change in output voltage.

where:

I! %X =percentage change in light input; and

-X10O=pereentage change in error output voltage,

ER based on output voltage, i,R which equals E at balance.

g 2.3Ea v (17) showing that phototube transfer gain is a function onlyof the reference voltage employed, andindependent of dynode voltage andof anode load resistance. f

I The dynode voltage controls the current amplification of thephototube, and therefore can be to control the light level at which thesystem stabilizes, according to the relations shown in Equations 6 and11 above, without affecting system gain. The dynode voltage determinesonly the cathode illumination, h", .required to produce a given anodecurrent, i

The value of dynode voltage applied to tube 22 (FIGS.

1 and 2) determines the light level at which the servo 15 systemstabilizes for a given negative density. This voltage is adjusted, bymeans of the present invention, to accomplish three control functions,viz: '(a) setting the aimpoint of the servo system, that is, setting theexposure to the desired value for a normal, standard or averagenegative; (b) shifting the aimpoint to introduce the factors previouslyidentified as negative classification and color correction; and (c)shifting the relationship of negative density to print exposure forintroducing the factor identified as slope control.

Negative classification and color correction- The shifting oftheaimpoint of ,the system to introducenegative classification andc'olor correction represented as follows, based upon Equation 5:

log H=d+(K'+n+c) where:

n=negative classification increment'in log units; and

c=color correction increment in logjnnits v Equation 18 shows that ifnegative classification and color correction are introduced, theirvalues are included in the log intensity of the light ative 12 (FIG. 1).

Circuitwise, the corrections "n transmitted through negand c' areapplied to the control phototube as incrementalchanges of its dynodevoltage. It is desired to obtain a given change in intensity for a givencorrective increment:

i dl If: E (19) Thus, for a desired ratio change in log light intensityat system balance, a certain ratio change in dynode voltage is required.Expressed logarithmically, Equation 19 becomes I! 10% (th 10g (hi) Wheretwo independently applied ratio changes in log light intensity arerequired for the two separate corrective effects (It and c),

da da la i where n and c are the corrective changes expressed in 6 logunits. Substituting Equations 23 and 24 in Equation 21 and combining,

: lo /mdm/m FIG. 3 shows the basic circuit employing a summing amplifier30 to achieve the relationship of Equation 28. At the summing point atthe amplifier input, the currents i and i must add to zero:

If the reference voltage, -E is made equal to the value of uncorrecteddynode voltage, E and the factors f=10 /m 32 g: IO /m (33) then Equation31 is the equivalent of Equation 28.

The feedback network of FIG. 3 is shown in block form as the n+0 network28 in FIG. 1, where the output of network 28 is applied to the dynodevoltage input of the control phototube 22 through summing amplifier 30.

In the full circuit diagram of FIG. 4, several modifications of thebasic circuit of FIG. 3 are made. Since the printer performs red, greenand blue exposures of the paper sequentially, individual potentiometernetworks are provided to allow setting the factors 1'- and gindependently for each color. The switching from the red to the green tothe blue network for each factor f and g may be accomplished by means ofa gang switch 33, which may simultaneously control the substitution ofeach filter 16 and 20 for another filter of a difierent color.

Additional amplifiers 36 and 38 are used to provide power for thenetworks in which the factors f and g are developed. Amplifiers 30, 36and 38, as well as an amplifier 40, which is employed as describedhereinafter, are preferably of the type known as operational amplifiersand described in Korn and Korn, Electronic Analog Computers, Chapter 5,McGraw-Hill, 1952.

Since the range of voltage output of the circuit to the phototubedynodes is not large compared to the normal unmodified dynode voltage, afixed voltage supply 31 is connected in the output of the summingamplifier 30, so that the latter amplifier is required to furnish only avoltage corresponding to the correction factors applied.

Slope control The third exposure modification, identified as slopecontrol, depends upon the integrated density of the negative. Itsrelationship to negative classification and color correction is bestshown in FIG. 5, in which log intensity of the printing light is plottedagainst negative density. The line a represents the operation of thebasic servo system, wherein the intensity of the printing light, oraimpoint, is maintained constant, regardless of the density ofzthe'negative. Theflinesa rreprcsentsihe aimpoint as it is incrementallyshifted when negative classification and color correction have beenconsidered. A particular value of negative density d isselectedas thatfor a normal negative, and the slope control factor is applied to thesystem as an over-correction or under-correction for negatives departingfrom d Line. a, shows the system response for a particular value ofover-correction, and line a; corresponds to some amount ofunder-correction.

The slope control factor is applied to the basic servo loop of theexposure control system'by deriving a signal that is dependent uponnegative density and applying an appropriate function of that signal tothe dynode-voltage input of the control phototube1-22 (FIGS. 1 and 4).The required signal, dependent upon negative density, may be taken fromthe printing lamp, whose log intensity is seen from Equation 5, above,to be proportional to negativedensity. This signal is derived by meansof a second multiplier phototube 32, which is arranged as a logarithmicphotometer, as disclosed in connection with FIG. 2 of US. Patent No.2,413,706, granted January 7, 1947, to N. R. Gunderson. The salientcharacteristic of photometer 32 is that: its output dynode voltage isrelated to the intensity of the light source substantially as follows:

n a 5 m log m) r e and e represent dynode output voltages correspondingto lamp intensities H and H, respectively; and m is a constant. 7

where Alinear approximation of Equation 34, sufiiciently accurate overthe range of operation of photometer 32, is

C C 11 log where:

C is a constant;

e =output of photometer 32 for a normal negative;

.and

H =intensity of printing lamp for a normal negative.

The output of photometer 32 is applied to a computing circuit comprisingan s network 34 (FIG. 1), wherein a multiplying operation occurs. Onemultiplying factor is the density of the negative to be printed,expressed as a deviation from the density of a normal-negative. Thesecond multiplying factor is the selected slope control valve anddetermines the degree of increase or decrease of the aimpoint as afunction of the deviation from normal density. Otherwise stated, thefirst multiplying factor determines the horizontal position of 11,; inFIG. 5, whereas the second factor determines the slope of the line a.

Referring to FIG. 4, the s network is seen to corn prise an analogmultiplier of the type disclosed and claimed in thecopending applicationSerial No. 756,585, filed August 22, 1958, by A. E. Glandon. Briefly,the multiplier comprises an input lead 35, which connects the output ofphotometer 32 directly to the input of summing amplifier 30 through aresistor-R Lead 35 also is connected tothe input of amplifier 30 througha series network comprising a rcsistor'R an amplifier '40 in parallelwith a resistor R a potentiometerR, and a resistor R The summingamplifier 30 may be considered as an output stage of the analogmultiplier, with the n+0 network constituting a feedback path parallelto amplifier 30. With this configurationin mind, the output'E of themultiplier is related to its input as follows:

where:

R is the equivalent resistance of the n+0 network for any settings offactors and g; and p is the ratio of output to input of potentiometer RThe setting of potentiometer R to adjust the value p determines theslope of the line a in FIG. 5.

The other multiplying factor, determining the horizontal positionof d inFIG. 5, is introduced as a reference potential e in the circuit shown inFIGS. 1 and 4. 1 Referring to FIG. 4, a source of potential e isconnected to the inputs of amplifiers 40 and 30 through a pair ofresistors R and'R respectively. The value of e determines the value of ethat produces a zero output from the multiplier, i.e., it determines thehorizontal position of d in FIG. 5. The equation for the multipliercircuit, including the input of e,, is as follows:

nc 3 Bn 6 no 3 ue (R.R. R. la-.121 R.)

The output of the "s" network is applied through the summing amplifier30 to the dynode input of the control tion as described hereinabove andas defined in the appended claims.

We claim:

1. In an exposure control system for a photographic printer, thecombination comprising: means for projecting the image of a transparencyonto a printing plane in which a sheet of light-sensitive printingmaterial is to be located; a variable-intensity printing lamp forilluminating said transparency to project an image thereof onto saidprinting plane; means for regulating the intensity of said lamp untilthe intensity of the projected image reaches a selected value andincluding a photomultiplier tube having a plurality of control dynodesand disposed for illumination by said lamp through said transparency,said photomultiplier tube being coupled to said lamp for controlling theintensity of said lamp; means for supplying control voltage to saiddynodes; means for generating a first signal which is a function of thedensity of said transparency; means for generating a second signalrepresenting the density of a standard transparency; means connected toboth of said generating means for comparing said first and secondsignals to generate a difference signal; means connected to saidcomparing means for multiplying said difference signal by a multiplyingfactor to generate a correction signal; and means interconnecting saidlast-named generating means and said control-voltage-supplying means forchanging said dynode voltage as a function of said correction signal,thereby changing said value of image intensity as a function of saidcorrection signal.

2. The exposure control system defined in claim 1, wherein said meansfor generating the first signal comprises a logarithmic photometeradapted to receive light directly from said printing lamp.

3. The exposure control means defined in claim 2, with manually operablemeans connwted to said multiplying circuit for selectively adjusting thelatter, to thereby selectively adjust said multiplying factor.

4. In an exposure control system for a photographic printer, thecombination comprising: means for projecting the image of a transparencyonto aprinting plane in which a sheet of light-sensitive printingmaterial is to be located; a variable-intensity printing lamp forilluminating said transparency to project an image thereof onto saidprinting plane; means for regulating the intensity of said lamp untilthe intensity of the projected image reachesaselected value andincludinga photomultiplier tube having a plurality of control dynodesand disposed for illumination by said lamp through said transparency,said photomultiplier tube being coupled to said lamp for controlling theintensity of said lamp; with means for supplying control voltage to saiddynodes; means for deriving a correction signal which is a function ofthe density of said transparency; a summing amplifier having an inputand an output; means for applying said correction signal to the input ofsaid summing amplifier; means for applying the output of the summingamplifier to the dynodes of said photomultiplier tube; and a multiplyingfeedback circuit inter-connecting the output of the summing amplifier tothe input of said summing amplifier, for multiplying said dynode voltageby a selected factor and thereby changing the intensity of saidprojected image as a function of said factor.

5. The exposure control system defined in claim 4, wherein said feedbackcircuit includes two series-connected sets of multipliers, each setcomprising three parallel-connected, manually settable potentiometers,and respective selector switches for selecting a potentiometer in eachset, with means for operating said selector switches in gang.

References Cited in the file of this patent UNITED STATES PATENTS2,561,243 Sweet July 17, 1951 2,757,571 Loughren Aug. 7, 1956 2,794,366Canaday June 4, 1957

